Simulated changes of the Southern Ocean air-sea heat flux feedback in a warmer climate

Abstract

In this paper, we have evaluated the Southern Ocean (SO) heat flux feedback in a fully coupled model and for the first time examined how this feedback evolves in response to global warming. The model broadly captures the observed characteristics of heat flux feedback over the SO. The heat flux tends to damp SST anomalies over the SO and thus the feedback is negative. In a warmer climate, the negative heat flux feedback in the SO, contributed mainly from turbulent component, becomes stronger. The turbulent feedback in the present day is primarily balanced by the upper boundary that strongly depends on background SST and wind and the thermal adjustment of boundary layer to SST anomalies. It is found that this balance shifts a little bit under global warming scenario. The upper limit increases in a warmer climate due to warm SST responses. The thermal adjustment of boundary layer becomes weaker in a warmer climate because of decreased atmospheric background heat convergence. The mean Deacon Cell transports anomalous heat caused by the greenhouse gas effect northward, leading to a heat convergence along the northern flank of the Antarctic Circumpolar Current. Constrained by the energy, the atmospheric northward heat transport has a corresponding divergence north of 55°S. This anomalous heat transport divergence favors air heat leaving away from 55°S–35°S regions to the polar region, leads to smaller air temperature tendencies in the local compared to the present day and therefore leads to a weakened thermal adjustment of boundary layer. Therefore, both changes in the upper limit and thermal adjustment of boundary layer contribute positively to the enhanced turbulent feedback in a warmer climate. The dynamic component due to changes in wind tends to compensate these two positive contributors, but its magnitude is too small to become a dominant factor.